1. Field of Invention
The invention relates to an apparatus for pyrometrically scanning surface temperatures of printed circuit boards, and to a method for operating said apparatus.
2. Description of Related Art
Printed circuit boards populated with electric or electronic components, also designated as electric printed circuit boards or printed circuits, are elementary constituents of virtually all technical apparatuses, from television sets through automobiles and aircraft to machine tools, and form the core components of every computer. There is always a very great deal dependent on the proper function of a printed circuit board. Faults on the printed circuit board therefore have to be found by rigorous tests and eliminated. For the purpose of testing various apparatuses and methods are known, which are employed individually or in combination.
Electric test apparatuses are known, which make contact electrically with the printed circuit board and apply test currents thereto. Functional tests are also carried out, during which the printed circuit board runs in the normal operating mode and is monitored in the process. Specific tests also serve only for examining individual components arranged on the printed circuit board.
Supplementary test methods also operate non-electrically, such as e.g. optical examinations by means of video cameras which can ascertain deviations from a good printed circuit board.
A further important test possibility is generic thermal testing, during which the surface temperatures of the printed circuit board are determined.
For this purpose, the printed circuit board is electrically energized, which generates heat in all the components. The temperature pattern can be detected and compared with that of a good printed circuit board. Deviations, for example in the case of an overloaded component, in the case of supply being lacking or in the case of a component being completely absent, can thus be found in a simple manner. It is also possible to test unpopulated printed circuit boards, in which the conductor tracks through which current flows exhibit different temperatures depending on material and geometry.
It is known, for the thermal testing of printed circuit boards to photograph the latter using a thermally sensitive digital camera, that is to say using an areal array of thermosensitive sensor pixels that directly yields an image. However, the sensitivity values in the temperature-sensitive pixels of such areal image sensors are not particularly good.
An individual specialized pyrometric sensor is better, which is commercially available for different temperature ranges and with very good sensitivities. Such pyrometric sensors are used e.g. in clinical thermometers for determining the temperature of the eardrum or for monitoring bread temperature in an oven, or else for monitoring the temperature of steel during forging. What is always essential here is the pyrometric measurement principle, that is to say the principle of measuring a surface temperature from a distance.
A pyrometric sensor sees an observation spot at a viewing angle, which, in one exemplary embodiment, produces an observation spot having a diameter of 1 cm at the observation distance of 6 cm. The desired overall image of the printed circuit board has to be assembled from a multiplicity of observation spots in a scanning process.
One apparatus of the generic type is disclosed in GB 1 375 121 where a pyrometric sensor is mounted in stationary fashion at a distance from the printed circuit board. Its viewing direction is deflected by a rotating mirror into a scanning line running over the printed circuit board. Transversely with respect thereto, the scanning movement is produced by the transport of the printed circuit board.
This known design enables area scanning using a pyrometric sensor. However, there are disadvantages owing to the fixed observation distance. Specifically, the latter leads to a fixed, that is to say invariable, resolution. That is unsatisfactory since either the scanning of large areas takes too long or, in the case of small-scale structures, the resolution is too coarse.
A further problem is that in the case of a sensor—observing at a diverging viewing angle—the size of the observation spot, that is to say the resolution, is dependent on the observation distance. If a flat surface is observed, then there is no problem, but there is a problem in the case of uneven surfaces, e.g. in the case of typical printed circuit boards which are populated with very thick components and where the temperature on the printed circuit board itself or on a very high component is intended to be determined alternatively. The resolutions are then different as a result of this change in the observation distance, which can in turn be a great disturbance. This can be designated as a depth of field problem.